`rustup doc` should use `info!()` and friends for console output

In #4986, we have:

```console
> rustup doc --serve
Serving docs at http://127.0.0.1:63995/index.html (press Ctrl-C to stop)
```

... of course this is based on existing logic:


```console
> rustup doc std::error 
Opening docs named `std::error` in your browser
```

... but I have the feeling that we should systematically use the logging system here.